1. Technical Field of the Invention
The present invention relates to a tire having a radial carcass reinforcement, which is intended to be fitted on heavy vehicles such as lorries, buses, tractors, trailers, etc., and relates more particularly to the crown reinforcement of such a tire.
2. The Related Art
Generally, the radial carcass reinforcement of the tire in question, which is formed of inextensible reinforcement elements made of metal or of aromatic polyamide, is surmounted radially by a crown reinforcement comprising a plurality of crown plies. In particular, the crown reinforcement includes a triangulation ply formed of metallic elements oriented relative to the circumferential direction by a large angle of between 45xc2x0 and 90xc2x0. The triangulation ply is, in turn, surmounted by two working plies formed of inextensible metallic reinforcement elements that are parallel to each other within each ply and are crossed from one ply to the next, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction. The working plies, which form the working reinforcement, are generally covered by at least one so-called protective ply, formed, in the majority of cases, of extensible metallic reinforcement elements, which are also referred to as elastic elements.
In the case of a tire for xe2x80x9cheavy vehiclesxe2x80x9d, a single protective ply is present. Its protective elements are generally oriented in the same direction and at the same angle in absolute value as those of the reinforcement elements of the radially outermost working ply, and generally less wide than the least wide, radially outermost working ply. Such crown reinforcements do not have sufficient endurance, in that, when the tires are running under high drift stresses, numerous breaks in the rubber appear and spread between the edges of the two working crown plies.
The commonly-owned French Patent 1 435 804, in order to avoid separation of the ends of the crown reinforcement when the tire is subject to high-speed travel, teaches arranging on either side of the reinforcement a narrow ply of cables forming an angle which may be between 40xc2x0 and 70xc2x0 with the cables of the carcass reinforcement. Such ply may be placed partially either above the crown reinforcement, or between the two working plies, or between the crown reinforcement and the carcass reinforcement. Any material desired may form the cables.
Japanese Application JP 63/125 406, in order to improve the uniformity, advocates the use of small reinforcement plies for the edges of the working reinforcement, such plies being formed of reinforcement elements made of inorganic fibre, such as carbon fibre or glass fibre, which are oriented parallel to the elements of the radially outer working ply.
U.S. Pat. No. 5,435,369, in order to guard against separations at the ends of the crown plies, teaches covering the ends of the two widest plies with additional small plies of reinforcement elements of material selected from among steel, aromatic polyamide, nylon, etc., and oriented relative to the circumferential direction by an angle which may be between 0xc2x0 and 55xc2x0.
The progress made in terms of length of wear life (number of kilometers travelled) of the xe2x80x9cheavy-vehiclexe2x80x9d tires, and also the possibility of easy, economical later recapping, requires a crown reinforcement of improved resistance to separation between the edges of working plies.
In accordance with the invention, a tire is provided which comprises a radial carcass reinforcement radially surmounted by a crown reinforcement formed of at least one working reinforcement formed of two working plies, having axial widths L1 and L2, respectively, of inextensible metallic reinforcement elements, which elements are parallel to each other within each ply and are crossed from one ply to the next at angles xcex11 and xcex12 of between 10xc2x0 and 45xc2x0 with the circumferential direction, and of at least one ply, referred to as a protective ply, of axial width L3, such that L3 less than L2 less than L1. The reinforcement elements of the protective ply are inclined in the same direction as the reinforcement elements of the radially adjacent working ply. On either side of the equatorial plane of the tire, in the axial extension of the protective ply, is arranged an additional ply of inextensible metallic reinforcement elements of the same direction as that of the reinforcement elements of the radially outer working ply and forming with the circumferential direction an angle xcex14 greater in absolute value than the angle xcex12 of the elements of the working ply by at least 5xc2x0. The axial width LA of the additional ply is such that its axially outer end is distant from the equatorial plane by an amount greater than half the width of the least wide and radially outer working ply, and such that it does not cover the axial end of the widest and radially inner working ply. The axially outer end of the additional ply is located at a distance from the equatorial plane of the tire of between 42.5 and 47.5% of the axial width L1 of the widest working ply which is radially to the outside.
As is known per se, the addition between the working reinforcement and the carcass reinforcement of what is called a triangulation ply may be advantageous, such triangulation ply being formed of inextensible metallic elements inclined greatly relative to the circumferential direction at an angle which may be between 45xc2x0 and 90xc2x0, and having an axial width L0 which is less than the width L1 of the first working ply.
Preferably the angular difference xcex14-xcex12 is not greater than 20xc2x0. If the difference is less than 5xc2x0, the resistance to separation between working plies is not improved; if the difference is greater than 20xc2x0, it is possible to obtain the opposite effect to that desired, that is to say, to have an adverse effect upon the separation between edges of the working crown plies.